renderergl2: reimplement greyscale as backend post after all 2D+3D draws

Fixes #654 - HUD/UI previously bypassed greyscale.
r_greyscale now supports partial desaturation [0-1].
Removed unused legacy logic from tr_shade.c

code/renderergl2/tr_local.h

@@ -708,6 +708,8 @@ typedef enum
 
 	UNIFORM_BONEMATRIX,
 
+	UNIFORM_GREYSCALE,
+
 	UNIFORM_COUNT
 } uniform_t;
 
@@ -1494,6 +1496,7 @@ typedef struct {
 	FBO_t *last2DFBO;
 	qboolean    colorMask[4];
 	qboolean    depthFill;
+	float       greyscale;
 } backEndState_t;
 
 /*
@@ -1614,6 +1617,7 @@ typedef struct {
 	shaderProgram_t ssaoShader;
 	shaderProgram_t depthBlurShader[4];
 	shaderProgram_t testcubeShader;
+	shaderProgram_t greyscaleShader;
 
 
 	// -----------------------------------------